KNIME essentials

What do red, yellow and green node states mean?

Red = not configured, yellow = configured and ready, green = executed with output available.

A previously green node turned red. Likely cause?

An upstream column it references was renamed or removed. Hover for the specific message.

What happens when you reconfigure a node?

It resets, along with everything downstream, because their inputs may now differ.

Why do postcodes lose leading zeros on import?

Type guessing reads them as numbers. Set the column to String in the reader Transformation tab.

Which node shows you the rows a filter excluded?

Row Splitter — non-matching rows come out of the second port.

How do Rule Engine rules evaluate?

Top to bottom, first match wins. Order most specific first and end with a TRUE catch-all.

In KNIME, what is a missing value?

A distinct state — not an empty string and not zero. Test with MISSING $col$.

How do you build a LEFT JOIN in the Joiner?

Include matching rows plus left unmatched rows in the output.

Joiner output exceeds the left input row count. Cause?

Duplicate keys on the right. Add a GroupBy on that branch, grouped by the join key.

Why does a downstream node fail to find "Revenue" after GroupBy?

GroupBy renames it to Sum(Revenue) by default. Set naming to keep original names.

Concatenate fails with duplicate row keys. Fix?

Let the node append a suffix or generate new RowIDs — KNIME requires unique RowIDs.

What does Column Appender match on?

Row position, not a key. Wrong unless the tables are genuinely parallel.

Why can a workflow break after a Pivoting node?

Column names are built from data values, so new values change the output schema.

Metanode versus component?

A metanode tidies the canvas. A component has its own scope, can expose settings, and is reusable across workflows.

How do you run a KNIME workflow on a schedule for free?

The batch executor from the command line, driven by cron or Task Scheduler.